1. Field of the Invention
This invention relates generally to a molding apparatus and, more particularly, to an apparatus selectively operable for either injection or transfer molding.
2. Description of the Prior Art
The molding of plastics in various forms has been known for many years wherein the specific apparatus used in the molding operation is generally dependent on whether the molding material has thermoplastic or thermosetting characteristics. Injection molding is the principle method of forming thermoplastic materials. In injection molding, plastic material is conveyed into a heated material feed cylinder, and a screw or plunger pushes the material through the cylinder where it is softened to a fluid or plasticized state. The discharge end of the injection cylinder terminates in a nozzle which communicates with the cavity mold supporting structure. The arrangement is such that the heated fluid plastic is discharged at high pressure through the nozzle and into the relatively cool mold via the conventional gates and runners provided in the cavity mold supporting structure. When the plastic material that is injected into the closed mold cools to a solid state, the mold is opened and the molded plastic article is ejected from the apparatus. In view of the thermoplastic characteristics of the material, any such material remaining in the injection feed cylinder, and in the runners of the cavity mold supporting structure, is reheated to a fluid state for injection into the mold during a subsequent cycle of operation.
Transfer molding is the most generally used method of forming thermosetting plastic materials. In transfer molding, the material is conveyed into an unheated material feed cylinder whereupon a plunger pushes the material through the cylinder and through a conventional transfer pot into the cavity mold supporting structure. The material is now softened to a fluid or plasticized state by means of the heat being applied to the cavity mold. The material is then forced by the plunger into the heated mold, under pressure, via the conventional gates and runners provided in the cavity mold supporting structure. The plastic is cured into an infusible state in the closed mold, whereupon the mold is opened and the molded plastic article is ejected from the apparatus. In view of the thermosetting characteristics of the material, it is important that substantially all such material in the transfer feed cylinder and in the transfer pot be forced into the cavity mold supporting structure to reduce the likelihood of any such material being cured in the feed cylinder or pot, and thereby preventing the transfer of additional materials in a subsequent cycle of operation. It will be appreciated that when the molded plastic article is ejected from the apparatus, conventional means are provided for also removing the cured material from the cavity mold runners so as to provide a free and unobstructed path for the thermosetting plastic material to flow from the transfer feed cylinder to the cavity mold in the next subsequent operating cycle of the apparatus.
In view of the curing of thermosetting material into an infusible state, it is not practical or economically feasible to injection mold such material. This is because the plastic material must first be softened to a fluid or plasticized state, by the application of heat, in order to flow through the injection nozzle. However, since heat temperatures and time factors are so critical, it is difficult to prevent some curing of the thermosetting material from taking place in the injection feed cylinder. This causes frequent stoppage or breakdown of the apparatus which, in turn, results in many lost hours of machine operability.
Since thermosetting materials cannot be allowed to remain in the injection feed cylinder long enough to set or cure, attempts have been made in the past to use a reciprocating screwtype feed cylinder with low heating capacity to liquify the thermosetting plastic material just as it goes through the injection nozzle. Very little heat, if any at all, is applied to the rest or major portion of the cylinder itself. Attempts have also been made to use the same reciprocating screw-type feed mechanism in a material feed cylinder selected for transfer molding of thermosetting plastic materials. This permitted, to some limited degree, the dual purpose use of a reciprocating screw-type feed mechanism for an injection feed cylinder with high cylinder heating capacity and for a transfer feed cylinder with low cylinder heating capacity.
The dual purpose reciprocating screw-type feed mechanism has several major drawbacks. The use of such mechanisms in transfer feed cylinders for thermosetting materials is limited to a relatively small variety of short fiber and plain thermosetting resins. The screw component itself is rather expensive and is usually designed for use with a specific resin material. Thus, it is necessary to have several screws on hand depending on the particular resin material in use. The screws are subject to harsh wear and must be replaced frequently thus contributing materially to high replacement costs. Furthermore, many costly productive hours are lost due to machine downtime in interchanging an injection material cylinder with a transfer material feed cylinder, and vice versa, which requires several hours of setup time by skilled mechanics. In addition, even the low heating capacity of the transfer material feed cylinder causes some curing of the thermoset resins which results in stoppage of the operating cycle.
The difficulties and drawbacks referred to above have led many users of such molding apparatus to rely on separate units to perform injection molding and transfer molding operations, respectively. Thus, it is common today to use a screw-type feed mechanism in a heated injection material feed cylinder for injection molding of thermoplastic materials, and a separate conventional plunger-type feed mechanism in an unheated transfer material feed cylinder for transfer molding of thermosetting materials. These separate units operate independently of each other and provide for good quality control of the molded article at relatively low manufacturing costs. However, it will be appreciated that a great deal of duplication is necessary in operating separate units. For example, each unit requires a frame for supporting the moveable and fixed platens which, in turn, support the cavity mold pieces. Each unit also requires its own dispenser for conveying the molding material from a hopper to the material feed cylinder. Separate operating controls are also necessary for the respective units. The duplication of components common to both injection molding and transfer molding units require greater investment of capital and larger manufacturing facilities, which in turn, usually results in increased operating expenses and overall higher manufacturing costs.
Accordingly, an object of the present invention is to provide a molding apparatus which is selectively operable for either injection molding of thermoplastic materials or transfer molding of thermosetting materials.
Another object of the present invention is to provide a molding apparatus having component parts which are common to the operation of the apparatus, regardless of whether the apparatus is in the injection mode or transfer mode of operation.
A further object and feature of the present invention is to provide a novel dual piston displacement cylinder having a selectively variable stroke depending on whether the molding apparatus is in the injection mode or transfer mode of operation.
Another object and feature of the present invention is to provide a dual purpose molding apparatus for a wide variety of thermoplastic and thermosetting plastic resins. The apparatus utilizes a plunger-type feed mechanism in either the injection mode or transfer mode of operation for improved quality control of the molded article, and thereby eliminates the costly reciprocating screw-type feed mechanism heretofore used in injection molding equipment.
A further object, feature and advantage of the present invention is to provide a relatively inexpensive dual purpose molding apparatus for either injection molding or transfer molding, and which can be simply changed from one mode of operation to another in a relatively short period of time.